The invention is based on the application of the known laser-Doppler-technique for measuring the superficial circulation of blood in cutaneous tissue. This technique is described, for instance, in U.S. Pat. No. 3,511,227, U.S. Pat. No. 4,109,647, SE 419 678 and in the articles "In Vivo Evaulation of Microcirculation by Coherent Light Scattering", Stern N. D., Nature Vol. 254, pages 56-58, 1975; "A New Instrument for Continuous Measurement of Tissue Blood Flow by Light Beating Spectroscopy", Nilsson, G. E., Tenland T. and Oberg P. .ANG.., IEE trans. BME-27, pages 12-19, 1980, and "Evaulation of a Laser Dopper Flow Meter for Measurement of Tissue Blood Flow", Nilsson G. E., Tenland T. and Oberg P. .ANG.., IEE trans. BME-27, pages 597-604, 1980. In principle, this technique involves directing a laser beam onto a part of the tissue and receiving, with the aid of an appropriate photodetector, a part of the light scattered and reflected by that part of the tissue irradiated by the laser beam. As a result of the Doppler effect, the frequency of the reflected and scattered light will be broadened and the light will thus have a frequency spectrum which is broader than the original laser beam, this broadening of the light frequency being due to the influence of the movement of blood cells in the superficial part of the irradiated tissue. The extent to which the frequency is broadened and the light intensity within different parts of this broader frequency spectrum constitute a measurement of the magnitude of superficial blood circulation in the irradiated part of the tissue examined and can be determined or evaluated by appropriate processing of the photodetector output signal.
An instrument for visually presenting blood flow is known from U.S. Pat. No. 4,862,894. In this known instrument, the reflected laser light is detected by a line sensor consisting of a plurality of light receiving elements which emit successively signals that are converted and stored in a memory. A microprocessor makes calculations based on these stored signals.
According to U.S. Pat. No. 4,862,894, the reflected Doppler signal is evaluated by measuring the amplitude of the signal at time t and comparing the result with the amplitude of the signal at time t+.DELTA.t. Thus, the amplitude values of the Doppler signal are detected by this method, whereas the frequency content of the signal is completely ignored. However, the signal processing technique and the use of the algorithm described in U.S. Pat. No. 4,862,894 enable the technique for scanning the measurement area described in this Patent Specification to be used.
Under some conditions, the signal processing technique described in U.S. Pat. No. 4,862,894 will result in erroneous evaluation of the blood flow in a body part, since the frequency content of the Doppler signal is a component which is necessary in order to describe the blood flow correctly. Perfusion is proportional to the product of the speed and concentration of red blood cells and it is not possible to evaluate perfusion or average speed unambiguously, solely by studying changes in the amplitude, as proposed in U.S. Pat. No. 4,862,894.